Curb Inlet Capacity Calculator (HEC-22)
Find how much gutter flow a curb-opening inlet intercepts on grade: enter the opening length and the flow depth at the curb.
Intercepted Flow (Weir Flow, On Grade)
| Item | Value |
|---|
How the calculation works
HEC-22 curb-opening inlet on grade (weir flow)
Qi (cfs) = Cw × (L + 1.8 × W) × d1.5
L = curb opening length (ft), W = depression width (ft), d = flow depth at curb (ft)
Cw = 3.0 for depressed gutters, 2.3 for undepressed gutters (US customary units)
Qi (cfs) = Cw × (L + 1.8 × W) × d1.5
L = curb opening length (ft), W = depression width (ft), d = flow depth at curb (ft)
Cw = 3.0 for depressed gutters, 2.3 for undepressed gutters (US customary units)
Notes & limitations
Equation and weir coefficients per FHWA Hydraulic Engineering Circular No. 22 (HEC-22), Urban Drainage Design Manual, Chapter 4. The effective weir length (L + 1.8·W) accounts for flow curving into the opening from the depressed gutter.
- Valid for inlets on grade operating as weirs. Inlets in a sag (sump) can transition to orifice flow at greater depths — use the HEC-22 sag-inlet equations for those.
- This is the theoretical interception; compare it with the gutter flow arriving at the inlet to find the bypass (carryover) flow to the next inlet.
- Clogging reduces real-world capacity — many agencies apply a clogging factor (e.g., 10–25% reduction) for design; confirm local practice.
- Keep the depression width consistent with the gutter section used in the gutter flow check.
← All water treatment calculators · Related: Rational Method Runoff Calculator